1. Field of the Invention
The present invention relates to a method for producing an organic electroluminescence element and a luminescent organic thin film obtained thereby, which are useful especially in, for example, organic semiconductor materials and organic electronic devices such as organic EL elements.
2. Description of the Related Art
By virtue of their various optical and electrical characteristics, organic semiconductor materials have recently attracted large attention as optoelectronics materials, and research and development thereon have actively been conducted. Among them, organic electroluminescence (hereinafter referred to as “organic EL”) elements, which are luminescent devices using luminescent organic semiconductors, utilize a phenomenon that when an electrical field is applied to an organic material, injected holes and electrons recombine with each other on an organic molecule to generate excitons, so that their radiative deactivation results in light emission. Such organic EL elements are expected to have applications such as display devices such as TV and mobile terminals and light sources for lighting.
Thus, various attempts have been made on develop techniques of making organic EL elements drive at low voltages and increasing organic EL elements in luminescent efficiency.
One exemplary organic EL element contains: a transparent electrode of, for example, indium-tin oxide (hereinafter referred to as “ITO”); a metal electrode of, for example, aluminum; and a plurality of organic thin layers between the electrodes.
The organic thin layers contain a luminescent material (a luminescent layer) and the luminescent material receives voltage via the transparent electrode and the metal electrode. When voltage is applied to between the transparent electrode and the metal electrode, holes are injected from the transparent electrode and electrons are injected from the metal electrode in accordance with the direction of the electrical field applied, and the electrons and holes recombine with each other in the luminescent material to thereby emit light.
The theoretical luminescent efficiency of organic EL elements is a product of light extraction efficiency, carrier recombination efficiency, exciton generation efficiency and luminescence quantum yield. In order to produce an organic EL element having such a high luminescent efficiency that is practically usable, it is an important technical object to increase the luminescence quantum yield of an organic luminescent dye used for the luminescent layer. Organic luminescent dyes show considerable degradation in luminescence quantum yield at high concentrations, which is called concentration quenching. This is because excitation energy is moved between closely existing molecules or light is absorbed by themselves (see Appl. Phys. Lett. 86, 071104 (2005)).
Thus, the luminescent layer of organic EL elements is generally in a solid state where an organic luminescent dye (guest) is dispersed at a low concentration in an optically inactive medium (hereinafter referred to as “host”). Here, host materials generally used are materials having an energy gap greater than that of the organic luminescent dye. Also, in the case of phosphorescent materials, hosts having levels higher than the lowest triplet level of the guest molecule are generally used in terms of efficiency.
The luminescent organic thin film may be produced by the following two methods, for example.
(1) Thin film of a vapor deposition/dispersion type: an organic luminescent dye serving as a guest material is dispersed in a host material of a low-molecular-weight material to form a luminescent organic thin film. The luminescent organic thin film is formed by, for example, the vacuum vapor deposition method (see Japanese Patent Application Laid-Open (JP-A) Nos. 2000-068057 and 2010-034484).(2) Thin film of a polymer dispersion type: an organic luminescent dye serving as a guest material is dispersed in a host material of a polymeric material to form a luminescent organic thin film. The luminescent organic thin film is formed by, for example, the coating method (see JP-A No. 2007-305783).
Any of these methods still have problems. First, the formation of the thin film of a vapor deposition/dispersion type requires a high-level technique of strictly controlling the vapor deposition rates of the guest and host molecules in order to properly control the concentration of the guest molecule doped. Moreover, in order to obtain white light as a light source for lighting, it is necessary to simultaneously vapor-deposit a plurality of organic luminescent dyes showing red, green and blue for doping, which requires strictly controlling the vapor deposition rates of the plurality of guest molecules as well as the vapor deposition rate of the host molecule. This method is quite difficult to perform and it is hard to say that its practicability and productivity are high. In addition, the development cost and time required for increasing the practicability and productivity thereof become considerable and thus this method cannot meet a requirement that an organic semiconductor material is formed into a thin film through a simple process such as coating or printing.
Meanwhile, the formation of the thin film of a polymer dispersion type uses the coating method and does not require strictly controlling the vapor deposition rates of the guest and host materials, which can simplify the production process. Also, wet film-forming processes such as the polymer dispersing method have the following advantages: it does not require the vacuum process; it can easily form a film having a larger area; and it is easy to mix a plurality of materials having various functions in a single layer (coating liquid).
At present, such wet film-forming processes, however, have difficulty in laminating layers and form elements poorer in drive stability than those formed by the vacuum vapor deposition method, and most of the wet film-forming processes are not at a practically usable level. The layer lamination by the wet film-forming processes is performed by forming the first layer using an aqueous solvent and a polymer insoluble to an organic solvent and forming the second layer thereon using an organic solvent. However, it is difficult to laminate three or more layers. Furthermore, the thin film of a polymer dispersion type brings about phase separation between the polymeric materials and the low-molecular-weight materials due to, for example, heating treatments, making it difficult to attain uniform dispersing. Also, the thin film of a polymer dispersion type is lower in luminescent efficiency than elements formed using low-molecular-weight materials. Needless to say, satisfying “uniformity” and “high luminescence quantum yield” at the same time is necessary for EL elements.